Magneto-inductive flow measuring devices utilize the principle of electrodynamic induction for volumetric flow measurement: Charge carriers of the fluid moved perpendicularly to a magnetic field induce a voltage in measuring electrodes likewise arranged essentially perpendicularly to the flow direction of the fluid. This voltage induced in the measuring electrodes is proportional to the flow velocity of the medium averaged over the cross section of the measuring tube; it is thus proportional to the volume flow.
In the ideal case, the electrical current curve in the coil arrangement corresponds to the curve of the magnetic field. Due to eddy currents, which arise during the reversal of the magnetic field in the pole shoes and cores of the coil arrangement, there occur in the real case deviations from the ideal case. The coil current measured outside of the coil arrangement corresponds, consequently, to the sum of the electrical current flowing in the coil arrangement and the electrical current produced by the eddy currents. If the electrical current measured outside of the coil arrangement is used as control variable, thus, indeed, the electrical current is constant, not, however, the magnetic field. This holds until the eddy currents have decayed.
In order to remove this drawback, it is provided in European patent, EP 0 969 268 A1 that the electrical current is not used directly for controlling the voltage across the coil arrangement. For rapidly reversing the direction of the magnetic field, an overvoltage is applied to the coil arrangement for a rise time during the reversing of the magnetic field. The duration the overvoltage is successively so set that the electrical current maximum is achieved upon expiration of the rise time, so that no further rise of the coil electrical current occurs. After reaching the maximum, the coil current asymptotically approaches the electrical current end value. In the solution known from the state of the art, the magnetic field has upon reaching the electrical current maximum a constant magnetic field end value corresponding to the constant electrical current desired value. The duration of the reversal phase is given by the characteristic of the coil current. Since the stability of the measurement signal is degraded by, among other things, also the inductive in-coupling from the coil arrangement to the measuring electrodes, during the measuring of the voltage difference between the measuring electrodes, both the voltage is across the coil arrangement as well as also the electrical current through the coils must be constant. In the case of the solution known from the state of the art, this is, due to the asymptotic approach to the end value, only the case once the eddy currents have completely decayed. In summary, the above cited EP 0 969 268 A1 describes an indirect control of the B-field by means of applying an essentially constant overvoltage.
Disadvantageous in the case of the above mentioned solution is that the rise time of the coil current can have a relatively strong dependence on process- and/or environmental conditions. In the case of a change of the rise time, unavoidably also the time between the end of the rise time and the beginning of the following measuring phase changes. Not completely decayed disturbance signals between the coil arrangement and the measuring electrodes, which are usually of a capacitive nature, influence the measured variables. The holding voltage depends on the resistance of the coil arrangement and on the desired electrical current. In such case, the holding voltage is defined as product of the resistance and the desired electrical current. Since the resistance is temperature dependent, in the case of constant overvoltage, the holding voltage changes. Since the overvoltage is usually not controlled, also fluctuations of the overvoltage can lead to an uncontrolled change of the rise time. As a result of the above mentioned influence of the environmental- and/or process conditions, fluctuations of the measured voltage difference between the measuring electrodes are experienced and, as a result thereof, fluctuations of the zero point of the magneto-inductive flow measuring device. Zero point instability lessens the accuracy of measurement and the reproducibility of the measurements of the magneto-inductive flow measuring device.